I think that depends on the meaning of the sentence. "Vi spiser fisk igen i morgen" means you are eating fish right now, and will do so again tomorrow - aka. "We eat fish again tomorrow". When you just want to say that you eat fish tomorrow, something you have already done in the past, or usually do on the weekday that is tomorrow, you'd say "Vi spiser igen fisk i morgen" - aka. "We again eat fish tomorrow" or "Tomorrow, we will again eat fish".